At the present time anesthetics (drugs which induce loss of sensation) are often used for surgical operations. A general anesthetic should cause a progressive depression of the central nervous system and cause the patient to lose consciousness. In contrast, a local anesthetic will affect sensation at the region to which it is applied.
Generally, the patient, prior to the operation, is anesthetized by a specialized medical practitioner ("anesthesiologist") who administers one or more volatile liquids or gases, such as nitrous oxide, ethylene, cyclopropane, ether, chloroform, halothane, etc. Alternatively, non-volatile drugs may be administered by injection or intravenous infusion, such drugs including pentothal, evipal and procaine.
Some of the objectives of a correctly administered general anesthetic are as follows:
First, the patient should be sufficiently anesthetized so that his/her movements are blocked. If the patient's movements are not sufficiently blocked, the patient may begin to "twitch" (involuntary muscle reflexes) during the operation, which may move or disturb the operating field (area being operated upon). Such blockage of movement occurs with a blockade of the neuromuscular junction with muscle relaxants and may also be accomplished by certain actions of anesthetic agents on the central nervous system.
Secondly, the patient should be sufficiently unconscious so as to feel no pain and be unaware of the operation. Patients have sued for medical malpractice because they felt pain during the operation or were aware of the surgical procedure.
Thirdly, the anesthesia should not be administered in an amount so as to lower blood pressure to the point where blood flow to the brain may be reduced to a dangerous extent (generally below 50 mm Hg for mean arterial pressure (MAP)), i.e., cause cerebral ischemia and hypoxia. For example, if the blood pressure is too low for over 10 minutes, the patient may not regain consciousness or be cognitively or otherwise impaired.
A skilled anesthesiologist may monitor the vital signals (breathing, blood pressure, etc.) of the patient to determine if more, or less, anesthetic is required. Often he/she looks into the patient's eyes to determine the extent of the dilation of the pupils or checks the lash reflex as an indication of the level (depth) of the effect of the anesthesia (called "plane of anesthesia"). However, there may be a number of problems with such complete reliance on the skill and attention of the anesthesiologist. In some operations, such as some heart surgery, the head is covered so that the patient's eyes cannot be viewed. Some operations may be prolonged, for example, 10 to 15 hours, so the attention of the anesthesia nurse or anesthesiologist may flag or fail.
It has been suggested that some of these problems would be avoided by having a computer system determine the best anesthetic mix and the amount of each anesthetic based on the patient's sex, age, weight, physical condition and the type of operation. However, it is believed that such a computer determination would not be successful due to the great diversity of response among individuals to different anesthetics, variations in premedications or mixtures of agents.
The inventor's prior U.S. Pat. No. 4,557,270, entitled "Electroencephalographic System For Intra-Operative Open-Heart Surgery", incorporated by reference herein, describes an electroencephalograph (EEG) system used intra-operatively in cardiovascular (open-heart) operations using a heart-lung machine (cardiopulmonary by-pass) such as heart valve replacement surgery. That system, called "CIMON" (Cardiovascular Intraoperative Monitor) is presently being sold by Cadwell Laboratories, Kennewick, Washington, and has been successfully used in many heart operations. However, the CIMON system, with its attention to the heart-pump rate, etc., is not used in general surgery.
In Chamoun U.S. Pat. No. 5,010,891 EEG potentials from a group of healthy surgical patients are recorded (col. 14, lines 32-34). A "reference array" is obtained of the most significant locations and an "autobispectral density index" is defined based on the recordings from a normal group. Each normal group index is then compared to the index of the patient under review. However, as explained above, the comparison of patients with a normal group, in itself, is not believed to provide reliable information in the surgical context of determining if a patient will be sufficiently anesthetized.